Hi Moose, and welcome,
I might be wrong (wouldn’t be the first time, by a long shot
), but it sounds like you’ve got archive scanning enabled (i.e., ZIP’s etc.). Avast needs to expand these into “real” temporary files before it can scan them, and of course that takes disk space.
I was never clear on at what point avast! deletes these temp files, whether as they’re completed or only after the complete scan is finished (which isn’t happening in this case because of the disk-space problem). I’d be happy to learn from one of the team (or anyone else who knows) more about how and when these temp files are cleared.
Try making sure the “include archives”, or however it’s worded, is not ticked and try it that way. If avast! doesn’t need to scan those, then it follows that it doesn’t need to expand them either.
Best,
Mike